To meet demand for obtaining a higher transmission capacity than a conventional satellite broadcasting system, DVB-S1, an increase in service availability through improved link margin, implementation of new services of broadband broadcasting such as HDTV, and appearance of a Ka-band satellite system, a DVB-S2 system, in which the DVB-S1 system is improved, is being developed. The DVB-S2 system applies an Adaptive Coding and Modulation (ACM) method to cope with effects caused by rain attenuation in a Ka band. In the ACM method, a transmitter changes adaptively coding/modulation methods and transmits Modulation Coding (MODCOD) information that is on the coding and modulation methods together with data according to data reception performance of a receiver, and the receiver changes decoding/demodulation methods of the received signal according to the MODCOD information.
A frame structure of the DVB-S2 system includes a BB frame that is an output of a stream adapter, a Forward Error Correction (FEC) frame that is an output of an FEC encoder and a PL frame that is an output of a physical layer (PL) framing unit. The BB frame largely includes a Base-Band (BB) header and a data field. In the FEC encoder, outer encoding of Bose-Chaudhuri-Hocquenghem (BCH) codes and internal encoding of Low Density Parity Check (LDPC) codes are performed, so that each parity is added to the BB frame to constitute the FEC frame. The PL frame divides the FEC frame block into slots of 90 symbols to transmit the classified results as a frame process for actual transmission through modulation. Start of Frame (SOF) that is information on startpoints of each frame, signaling information of MODCOD informing a transmission method and a pilot signal are inserted to constitute the PL frame. FIG. 1 schematically illustrates the configuration of a conventional DVB-S2 frame.
According to the conventional DVB-S2 system, the FEC frame formed by the BCH and LDPC encoding has a specific length (for example, 64,800 bits or 16,200 bits) regardless of a coding rate and a modulation method, and the length of the PL frame is variable depending on the modulation method.
As described above, according to the conventional DVB-S2 system, the length of the PL frame is changed depending on the modulation method and the coding rate, and it is difficult for a receiver to maintain initial frame synchronization to cope with the change.